Land development often increases the peak flow rate of storm water runoff as compared to the peak flow rate before development. FIG. 1 illustrates the flow of storm water runoff for a specific rain event on undeveloped land (shown in phantom lines as curve U) as compared to the flow of storm water runoff for the same rain event on the same land after development (shown in dotted lines as curve D). The peak flow rate of the undeveloped land is Q, while the peak flow rate of the developed land is increased to Q′.
Many jurisdictions require that storm water runoff of developed land be managed so that the peak flow rate for the specific rain event does not exceed Q, the peak flow rate of the undeveloped land.
A storm water management system manages storm water runoff from developed land. A storm water system receives the storm water runoff, and is designed to limit the maximum discharge rate for the specific rain event to a particular Q, often the pre-development peak discharge rate.
During the period of the rain event in which the rate of storm water flow into the storm water system exceeds Q, the storm water system must store the excess runoff for discharge later.
An ideal storm water management system would minimize the volume needed to store the excess runoff. An ideal storm water management system would initially discharge runoff at the same rate as the inflow rate into the system. A graph of the discharge rate of the ideal storm water management system would follow the flow rate of curve D of FIG. 1 from the onset of the storm until the intake rate reaches Q. After that point, the discharge rate of the ideal storm water management system would remain a constant Q (shown as the solid line C in FIG. 1) during the period while the inflow rate peaked at Q′ and then dropped back down to Q. After that point the ideal storm water management system could begin to empty and discharge runoff at a rate at or below Q (the emptying of the system is shown as solid curve E in FIG. 1).
While the intake rate remains above Q, the volume of runoff represented by the area A in FIG. 1 must be stored for later release. The release of stored runoff makes up the area B shown in FIG. 1.
Storm water management systems often use a flow control device that includes a storage chamber to receive and store runoff, the storage chamber provided with a discharge orifice of fixed size to discharge runoff from the storage chamber. The orifice is sized for a discharge rate of Q when the storage chamber is full. Such storm water management systems do not approach the performance of an ideal storm water management system because they discharge water at a less than optimum rate when filling up.
Other storm management systems utilize multiple fixed orifices at different elevations or divert excess intake into other storage chambers. Although these systems may better approach the ideal system than a simple fixed orifice system, their performance may still be lacking or their costs prohibitive for many projects.
Thus there is a need for an improved storm water management system that more closely approaches the performance of an ideal storm water management system.